DYRKs are a novel subfamily of dual-specificity kinases, and emerging developmental regulators. In yeast, YAK1P (the first described DYRK kinase) inhibits proliferation. In Dictyostelium, YakA is required for cell cycle arrest and terminal differentiation. In Drosophila, MNB kinase regulates neuronal development. In humans and mice, four dyrk kinase genes exist. While each is expressed in germline tissues, DYRK1 (MNB homologue) also is neuronally expressed, maps to Down syndrome critical region, and precipitates learning defects when expressed in transgenic mice. We now have discovered that DYRK3 is expressed selectively in erythroid progenitor cells; have cloned and characterized dyrk3 cDNAs and the murine dyrk3 gene; have developed kinase loss- and gain-of-function mutants; have discovered a connection between DYRK3 and PKA; have developed a transgenic model for studies of DYRK3 action; have described a defect in phenylhydrazine- stimulated erythropoiesis in these mice; have developed recombinant retroviruses encoding DYRK3 constructs; and presently are preparing dyrk3 -/ mice. Based on this progress, experiments are proposed to advance our understanding of DYRK3 biofunction in the following contexts: # 1) In pA2 gata-DYRK3 transgenic mice, at what stages and via what mechanisms does DYRK3 repress erythroid development?; #2) In what ways is erythropoiesis affected in marrow-derived (and ES) progenitor cells by retrovirally expressed DYRK3, constitutively active DYRK3, kinase-dead DYRK3 and/or DYRK3 antisense transcripts?; #3) In what ways are erythropoiesis (and hematopoiesis) affected due to gene disruption in dyrk3 -/"mice?; #4) In cell line and primary cell models, via what mechanisms does DYRK3 engage PKA, and how might DYRK3 regulate hematopoietic progenitor cell survival, growth and/or differentiation?; #5) What factors regulate erythroid specific DYRK3 expression and activation? These studies should provide insight into DYRK3's biological roles and action mechanisms, and into the actions of other mammalian DYRKs. DYRK3 also may emerge as an important target for small molecule inhibitors aimed at treating anemias associated with chemotherapy and chronic disease.